This invention relates to a structure formed of alternate thin layers of a superconductor and an insulating semiconductor. In particular, it relates to a structure formed by disposing a plurality of alternate layers of niobium nitride and aluminum nitride that exhibits improved superconducting properties at temperatures near absolute zero.
Superconducting materials in thin films are useful as metal-insulator-metal (MIM) sandwich structures such as Josephson Devices, formed by two layers of superconducting film separated by a thin insulating layer, or as an artificial superlattice (AS), which is a stack of superconducting films separated by insulating layers. Thin films are typically tens or hundreds of Angstroms in thickness. The term "thin" means less than the characteristic penetration depth of a magnetic field into the superconductor. In general, both the parallel critical field and the critical temperature are functions of the thickness of the superconductor, the materials of which it is made, and the uniformity of the crystal structure up to its surface. The parallel critical field is the strength of the magnetic field parallel to the layer at which superconductivity disappears for a given temperature. The critical temperature is the highest temperature at which a material exhibits superconductivity. To make a Josephson junction, it is necessary not only to have a superconducting material, but also to have an insulator that bonds well to the superconductor, that has high resistivity, and that can be deposited uniformly.
The deposition of alternate thin films of superconducting material and insulating material allows the fabrication of an artificial superlattice (AS) that forms a superconductor with enhanced parallel critical fields and high critical currents. Such structures are useful as current carriers for magnets in fusion applications. To optimize these properties, it is desirable also to maintain the crystal structure up to the surface of the superconducting material, with minimum roughness at the interfaces.